Several optical methods are currently used for measuring thickness of thin films in semiconductor workpiece (e.g. wafer) manufacturing, mainly based on spectrophotometry and ellipsometry. There are different configurations of optical measuring tools: in-situ, performing measurements during the processing; stand-alone (SA) tools having individual cassette-to-cassette load/unload means and integrated tools, installed within or adjacent to processing equipment for layer deposition (like CVD cluster tools of Applied Materials. USA) or for layer removal (like CMP polishers of Applied Materials, EBARA, SFI, etc.) Such integrated metrology tools provide accurate measurements of each wafer immediately after the processing, e.g. layer deposition/removal. The examples of such integrated metrology (IM) tools are NovaScan 840 of Nova Measuring Instruments, Israel, NanoSpec 9000 of Nanometrics, USA, etc.
Both SA and IM tools apply precise optical measurements, mainly based on spectral reflectometry, to the predetermined small measurement sites within the wafer's pattern structure. In order to reach this measurement site, the pattern recognition technique based on image acquisition and processing is applied accompanied by precise positioning and auto-focusing.
In order to measure the layer thickness distribution (uniformity) over the whole wafer, such pre-measurement cycle of pattern recognition, precise positioning and spectral measurement is repeated in number of measurement sites over the wafer, e.g. 25 points along the wafer of diameter=300 mm. Performance of entire measurement cycle, i.e. including pre-measurement cycle require significant amount of time and in some cases are the bottle-neck of the equipment run rate.
The main advantage of the above technique is the use of pre-determined measurement sites with known stack structure, so measurement data interpretation is done on a basis of accurate optical model of measured thin film structure, providing ultimate accuracy and repeatability of the film thickness determination.
In some case, it is more important to reach a maximal throughput (minimal measurement time) on account of some loss of measurement accuracy. For example, when the film thickness is relatively high (several thousands of Å) the accuracy and repeatability of single Å of existing IM tools may be not so needed. However, the higher measurement speed of several seconds for multiple measurements on wafer is very important.
For such cases so-called “large spot” measurement techniques are being developed. Systems using “large spot” do not require pattern recognition and precise positioning means. This technique is based on extracting useful average information about the top layer from mixed spectrum received from different parts of wafer pattern with different layer structures. Such techniques are disclosed for example the in U.S. Pat. Nos. 5,872,633: 5,900,633 and PCT patent Application No. PCT/IL99/00466 (Publication. No. WO 00/12958)
Being limited in measurement accuracy and repeatability, such technique still is useful for some applications. The main advantage of this technique is high measurement speed because such procedures as wafer alignment; pattern recognition and precise positioning are not needed. Moreover, applying a relatively large spot of about 20 mm, the light intensity reaching the photodetector is high and the integration time of photodetectors needed for required SNR (signal-to-noise ratio) is very small about 1-2 orders of magnitude smaller than for “small spot” IM or SA tools, where the integration time for single measurement may be as large as 1 second or the like.
The main limitation of the “large spot” technique is its low spatial resolution. It is especially critical for cases of high non-uniformity within a relatively small wafer's area.
For example, after CMP processing of silicon oxide layer, a high non-uniformity of the film thickness, especially in the wafer edge areas is observed. Thus, large spot of about 20 mm and larger can not provide needed information, when resolution of about 1 mm is needed. Reducing the spot size to the level of 1 mm will deteriorate drastically the measurement accuracy and in many cases makes this measurement impossible because of too strong dependence of received spectrum on the pattern structure.
Thus, there is a need in the art for measurement technique providing advantages of “large spot” and “small spot” techniques.